A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen, an alcohol (such as methanol or ethanol), a hydride or formic acid, is supplied to the anode and an oxidant, e.g. oxygen or air, or other oxidant such as hydrogen peroxide is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electro catalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of the oxidant at the cathode.
Fuel cells are usually classified according to their electrolyte: proton exchange membrane (PEM) fuel cells including hydrogen (including reformed hydrocarbon fuel) fuel cells, direct methanol fuel cells (DMFC), direct ethanol fuel cells (DEFC), formic acid fuel cells and hydride fuel cells; alkaline electrolyte fuel cells; phosphoric acid fuel cells (including hydrogen or reformed hydrocarbon fuel); solid oxide fuel cells (reformed or unreformed hydrocarbon fuel); and molten carbonate fuel cells (hydrogen and reformed hydrocarbon fuel).
In proton exchange membrane (PEM) fuel cells, the electrolyte is a solid polymeric membrane. The membrane is electronically insulating but ionically conducting. Proton-conducting membranes are typically used, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to create water.
The principle component of a PEM fuel cell is known as a membrane electrode assembly (MBA) and is essentially composed of five layers. The central layer is the polymeric membrane. On either side of the membrane there is an electrocatalyst layer, containing an electrocatalyst, which is tailored for the different requirements at the anode and the cathode. Finally, adjacent to each electrocatalyst layer there is a gas diffusion layer. The gas diffusion layer must allow the reactants to reach the electrocatalyst layer, must allow products to be removed from the electrocatalyst layer, and must conduct the electric current that is generated by the electrochemical reactions. Therefore the gas diffusion layer must be porous and electrically conducting.
The electrocatalyst layer is generally composed of a metal, (such as a platinum group metal (platinum, palladium, rhodium, ruthenium, iridium and osmium), gold or silver, or a base metal) either unsupported in the form of a finely dispersed metal powder (a metal black) or supported on an electrically conducting support, such as high surface area carbon material. Suitable carbons typically include those from the carbon black family, such as oil furnace blacks, extra-conductive blacks, acetylene blacks and graphitized versions thereof. Exemplary carbons include Akzo Nobel Ketjen EC300J, Cabot Vulcan XC72R and Denka Acetylene Black. The electrocatalyst layers suitably comprise other components, such as ion-conducting polymer, which is included to improve the ionic conductivity within the layer. The electrocatalyst layers also comprise a certain volume fraction of porosity, which allows reactant ingress and product egress.
Platinum black may also be used as a catalyst in a fuel cell anode or cathode. Platinum black is black-colored finely divided form of unsupported metallic platinum. Platinum blacks may be formed by a variety of methods including by heating ammonium chloroplatinate in molten sodium nitrate at 500° C. for 30 minutes. The molten mass is poured into water, followed by boiling and washing to give a muddy brown precipitate (thought to be platinum dioxide) that can be concentrated by centrifugation (Platinum Metals Rev. 2007, 51 (1), 52). Reduction of the suspension in water with gaseous hydrogen gives a black suspension with a particle size from which layers are made in the range of 0.5 μm to 10 μm (see FIG. 1). Typically, catalyst layers prepared from platinum black have a platinum loading of at least 1 mg/cm2.